The present invention relates to the field of internal combustion engines. It relates to a method for controlling the fuel supply to an internal combustion engine, in particular to a two-stroke motor.
In motor apparatus such as for example a chain saw usually the highest rotational speed is limited by overenriching the air/fuel mixture, which leads to spark failures (known under the name xe2x80x9cfour-strokingxe2x80x9d). Another way is the direct influencing of the combustion via an ignition cut-off on reaching a defined (maximum) rotational speed. These variants without additional measures however lead to the fact that unburnt fuel leaves the motor. This in turn not only directly leads to an undesired high emission of hydrocarbons, but also indirectly limits the possibility of the application of a catalytic converter, since the unburnt fuel contributes to overheating and thus to the destruction of the catalytic converter.
In order to exclude such overenrichment, in the automobile branch one has applied rotational speed limitations by way of fuel cut-offs. These may be realized in the manner of a controllable intake-manifold injection or also by way of a carburetor control via a nozzle cut-off valve. In motor apparatus with two-stroke motors however stored quantities of fuel in the crank housing and intake pipe lead to an undesired hysteresis about the governing rotational speed. With this, rotational speed fluctuations of approx. xc2x1700 1/min occur.
From DE-Al-39 11 016 there is known a method for operating a two-stroke internal combustion engine with which the fuel injection on idling and in the low part-load region is effected intermittently, wherein the fuel is injected during a constant angle of opening with a constant volume flow. If the rotational speed changes thus e.g. drifts away, the time during which the rotational angle is run through changes, and specifically inversely proportional to the rotational speed. From this there results a self-regulation of the rotational speed in that specifically with an increase in the rotational speed the quantity of injected fuel recedes or with a fall in the rotational speed the time duration of the fuel injection is increased and thus the quantity of the fuel injected with a constant volume flow increases. Such a direct self-regulation acting without a time delay is however only possible when the fuel is injected directly into the cylinder in an electronically controlled manner.
In DE-Al-199 21 020 there is described a method and a device for control of the fuel supply with a model motor with a fuel injection into the crank housing, with which the injection period required for a demanded quantity of fuel per cycle is determined in each case. If with low rotational speeds the determined injection period drops below a given minimal injection period, the continuous injection is switched over to an intermittent injection, wherein the injection data for the intermittent injection is deduced from a stored correspondence table. With high rotational speeds a hysteresis may also not be avoided here.
EP-131-0 715 686 discloses a method for controlling an internal combustion engine in which the air/fuel ratio is continuously automatically adapted to the operating condition, in order for example to achieve a maximal efficiency, maximum exploitation of the fuel and a rotational speed limitation. For this, periodically for a short time, e.g. by cutting off the supply of fuel, the air/fuel ratio is changed and the influence of this change on the rotational sped is measured. Additionally, the rotational speed which is not yet influenced, shortly after the change, and the influenced rotational speed after the effect of the change, i.e. after a few revolutions, are measured and compared to one another. The result of this comparison is used for optimizing the air/fuel ratio via a setting device. With this type of control the hysteresis is considerable.
It is therefore the object of the invention to specify a method for the control of the fuel supply of an internal combustion engine, which is simple to realize and largely avoids hysteresis effects with the parameters of the machine influenced by the supply of fuel, such as e.g. the rotational speed.
The core of the invention lies in the fact that the fuel supply in each case is already switched over (switched-on or cut-off) before the desired value of the parameter is reached coming from above or below. By way of this, fluctuations about the desired value are limited to a (narrow) band which may be set, and the hysteresis is largely compensated.
The method is particularly simple when according to a preferred formation of the invention the upper or lower threshhold value are equally distanced from the desired value. This above all is the case when the dependence of the parameter on the fuel supply is largely linear in the region of the desired value.
Preferably the parameter is the rotational speed of the internal combustion engine. The rotational speed fluctuations are limited to a small rotational band by way of the control according to the invention, by which means the rotational speed hysteresis is limited and the emission of hydrocarbons is kept small.
This is particularly the case when according to a further formation of the invention the desired value of the rotational speed is the desired governing rotational speed.
A help with the governing of the rotational speed may furthermore be achieved if additionally to the cutting-off or switching-on of the fuel supply, the ignition sparks for igniting the fuel/air mixture is switched off or on.
It is however also conceivable for the desired value of the rotational speed to be the idling rotational speed. A switch-over to the idling rotational speed as a desired value is particularly simple when the internal combustion engine comprises a throttle valve which on idling of the machine assumes an idling position, and if one takes the idling rotational speed as the desired value of the rotational speed when the throttle valve is located in the idling position.